Kat is dependent on Matilda physically, because she's the one literally keeping Kat alive with her magic. Kat needs Matilda in order to continue being a person. Which does bleed into an emotional aspect as well, but unfortunately a more manipulative one. Kat needs Matilda, so she'll make sure Matilda never has to leave her. Never has to go.
And Matilda is dependent on Kat emotionally. Without Kat (at least before getting close to Ichabod and closer to Brom) Matilda has nothing. Kat has always been the cool and popular one, the one with friends, the one who goes to parties. Matilda only dragged behind her, doing whatever Kat did. Because they were friends. Matilda needs Kat. She has to make sure Kat always finds her useful. Has to make sure Kat never has to leave her behind.
